A Hardware Multichannel Simulator for Wideband Wireless Systems
Hardware-based multipath fading simulators have traditionally been used to generate up to two simultaneous fading channels. Mobile network testing and future wireless applications like geolocation, smart antennas and multiple input multiple output (MIMO) systems, however, require more channels. With the advancement of mobile multimedia systems, required data rates and system bandwidths are increasing, and the development of such systems puts demands on the associated test equipment to have increased features and performance. Future radio channel simulators will have to have multiple channels, wide bandwidth, high dynamic range, a sufficient number of fading paths, advanced channel modeling and very high RF performance. Offering eight fading channels, 70 MHz RF bandwidth and spatial channel modeling, the PROPSim C8 wideband multichannel simulator has been designed to meet these requirements.
Fig. 1 The RF simulator's functional block diagram.
PROPSim C8 is a hardware multichannel simulator, where a maximum of eight independent channels are run in one simulator unit with more channels possible if multiple simulators are synchronized together. Applications, when testing antenna diversity, include carrier-to-interference (C/I), adaptive antennas, geolocation systems, handover, repeaters or other multi-antenna or multi-terminal systems. Also, MIMO systems use multiple antennas both in transmission and reception.
An important feature of the simulator is that because it is independent of the incoming signal it provides a very versatile platform for different tests. Any signal can be connected to the input when the RF bandwidth is 70 MHz or less, center frequency is between 350 MHz and 6 GHz, and the RF power is below 0 dBm. The hardware performs real-time simulation, digital path and digital channel combining, providing accurate and realistic real-time radio channel simulations.
Fig. 2 Simulator plug-in units.
The multichannel simulator provides three simulation interfaces: RF, analog baseband (ABB) and digital baseband (DBB). Regardless of the selected interface, however, multipath fading simulation, signal combining and splitting are done in the digital domain to achieve the best possible accuracy, flexibility and repeatability. A block diagram of the function is shown in Figure 1 . The signal is downconverted from RF to analog baseband (I and Q branches), transferred into the digital domain by an analog-to-digital converter (ADC) and vice versa via a digital-to-analog converter (DAC). The digital baseband processing performs very high speed multipath fading simulation and the faded analog baseband signal is upconverted back to RF.
Fig. 3 RF, ABB and DBB interfaces.
In addition, a hardware simulator is implemented by removable plug-in units (see Figure 2 ), utilizing a simulator controller unit (SCU), where an internal PC is installed. The baseband unit (BBU) consists of the ADC and DAC, along with digital baseband processing, and multipath fading is implemented in the digital domain. The BBU has two interfaces, ABB and DBB, while the RF unit (RFU) makes quadrature down- and upconversions, and provides the RF interface.
As referred to earlier, the system architecture supports three different simulation interfaces, as shown in Figure 3 . Represented are the transmitter (TX) under test or the test signal generator, the radio channel simulator (RCS) and the receiver (RX) under test. A typical transmitter has DBB components, a DAC and an upconverter (UC). In the receiver there may be a downconverter (DC), ADC and DBB. Similar parts can be found in the RCS, which facilitates the use of different interfaces. The use of the three interfaces brings the same radio channel used in the lab through the whole development cycle.
Also, the DBB interface extends the use of fading simulation to a very early phase of product development when analog parts are not available. It can be used as an accelerator of software simulation, non-real-time field-programmable gate array (FPGA) testing or testing different parts, such as application-specific integrated circuits (ASIC). Consequently, it helps to improve the quality of product design and reduces the time-to-market and cost of product development. When all three interfaces are in the same product, similar channel models are available in all phases from early algorithm design to final product tests. These phases can be non-real-time macro model, ASIC, analog baseband and RF performance tests, together with system verification and type approval.
Versatile channel modeling is required to ensure that the performance of the system is adequate in all situations. Testing only with models defined in various standards is often not sufficient to guarantee that the terminal actually operates in difficult fading environments. The testing of wireless products with scenarios that stretch requirements beyond type approval models is important in all phases of the product development cycle. Existing standards do not model the spatial dimension of the radio channel.
Fig. 4 Incoming radio wave received by a five-element antenna array.
Fig. 5 Antenna array application.
Advanced channel modeling software will help to design realistic scenarios and the use of the spatial dimension sets new requirements for radio channel simulators. For example, it can be utilized to improve system capacity. Multiple frequency selective fading channels with controlled correlation must be produced and the PROPSim C8 uses two alternative methods to implement these spatial requirements: correlation matrix and geometric constellation. The first method uses an operator selected set of mutual correlation values between the channels, while the second utilizes antenna array and direction of arrival (DoA) information to determine the correlation between channels.
This geometric constellation-based method is shown in Figure 4 . The modeling method assumes that the source is so far away that the received wave is a plane wave and the angular spread of the incoming wave follows Laplacian distribution. Zero angular spread will lead to a situation where phase shifts between antenna elements stay constant during simulation.
Fig. 6 Multiple terminals and basestation.
Fig. 7 MIMO system.
Typical applications for this multichannel simulator are different antenna array systems, mobile networks, MIMO systems and geolocation applications. Figure 5 shows a typical test setup for antenna array tests where signal combining and splitting is done digitally. Applications illustrated in Figures 6 and 7 are multiple terminals and basestation, and a MIMO system, respectively, with the latter being planned for use in upcoming third- and fourth-generation units. Another feature of the simulator is that each channel has an integrated digital noise source, whereby additive white Gaussian noise is generated internally and added to the faded signal. Typical wireless test systems require transmitter, channel, noise and receiver, so a combined noise source and fading channel simplifies the test setup.
With its multichannel capability, wide RF bandwidth and three simulation interfaces, the PROPSim C8 fading simulator offers enhanced features and performance to produce accurate and realistic simulations, and has been developed to provide the flexibility and adaptability needed to meet the requirements of future wideband wireless systems.
Elektrobit, Kauniainen, Finland, +358-400-308 396.
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